The invention relates to a method of manufacturing a semiconductor device, in which method a pattern is etched by a dry etching process, for example, a plasma etching process, in a substrate protected by a resist pattern which is the negative of the pattern to be etched.
As integrated circuit technology has developed, there has been a progressive increase in the density of the circuitry per unit area and the circuit geometry detail has become progressively finer, so that in some cases line-widths of less than 1 .mu.m are now required. Both in order to produce such fine geometries satisfactorily and also to avoid problems which are caused by residues of reagents and reaction products remaining on semiconductor structures which have been processed by wet chemical etching, dry etching methods are being used on a growing scale for etching semiconductor structures.
When dry etching methods have been used in the manufacture of semiconductor devices and positive-working resists having high sensitivities have been used to define the patterns to be etched, it has been found that the resist patterns were very rapidly eroded and therefore could not adequately protect the underlying semiconductor structures. This was not unexpected, because a positive-working resist is formulated so that it should degrade readily when exposed to ionising radiation, so that it would not be expected that a positive-working resist material would withstand degradation when placed in a plasma which is an environment which contains electrons and other charged species. Since some negative-working resists have a greater resistance to erosion by dry etching than positive-working resists, there is less need to increase the resistance to erosion by dry etching for negativ-working resits than there is to take this step for positive-working resists.
U.K. Patent Specification GB No. 2,121,197A describes a method of forming on a substrate a mask resistant to plasma etching, including the step of lithographically patterning a film of radiation-sensitive resist present on said substrate characterised by the further steps of providing a coating layer of material on the patterned resist film and on exposed areas of the substrate, effecting a heat treatment such that the material of said layer reacts chemically with the resist to form a plasma etch-resistant skin at the surface of the patterned resist film, and removing the unreacted material of the coating layer to leave the patterned resist film with the skin as the mask on the substrate. There is no mention in the GB No. 2,121,197A specification of any value of the sensitivity of the resist, and the sensitivity of a resist formed from a mixture of poly-(methyl methacrylate-co-methacrylic acid) and poly-(methyl methacrylate-co-methacryloyl chloride) when irradiated with 20 kV electrons is at least 10 .mu.C/cm.sup.2. When a charge density of only 10 .mu.C/cm.sup.2 is used to form a pattern in this resist, there is such a large reduction in thickness of the unirradiated areas of the resist film during development that the definition of the resist pattern is significantly impaired.